Process of annealing aluminum.



UNITED STATES PATEN OFFICE.

LAURENCE H. WHITNEY, 0F PITTSFIELD, MASSACHUSETTS, ASSIGNOB TO GENERALELECTRIC COMPANY, A CORPORATION OF NEW YORK. I

PROCESS OF ANNEALING ALUMINUM.

No Drawing.

To all whom it may concern:

Be it known that I, LAURENCE H. WHIT- NEY, a citizen of the UnitedStates, residing at Pittsfield, in the county of Berkshire, State ofMassachusetts, have invented certain new and useful Improvements inProcesses of Annealing Aluminum, of which the following is aspecification.

The present invention relates to they annealing of aluminum after themetal has been rendered hard by mechanical working, and the object of myinvention is to produce aluminum articles in which the unavoidablesilicon impurity is substantially all in the dissolved'state instead ofbeing present as graphitoidal silicon. The product of my invention is ofparticular utility in electrolytic cells, such as lightning arresters,but in its use is not necessarily limited to such devices.

Aluminum unavoidably contains about 0.20% of silicon. When aluminum isfused and cast the greater part of the silicon is found to be present inthe dissolved state. Upon mechanical working of the cast metal intodesired shape, it is hardened so as to require annealing. I have foundthat by annealing the metal as heretofore practised,

a large percentage of the previously dissolved silicon is converted intothe graphitoidal state. The graphitoidal silicon in the aluminum isdeleterious to the formation of a film on the aluminum when used aselectrode material for electrolytic cells, such for example, aslightning arresters. Chemically, this conversion is distinguishablebythe dark color of the residue from dehydrating the dissolved sample incertain mineral acids. This residue contains the graphitic or metallicsilicon. and silicon dioxid. From this residue the metallic siliconcanbe determined by a well-known electrochemical method.

In accordance with my invention, I have produced annealed aluminum bysubjecting the metal to a temperature intermediate between about -370 C.and 450 C. until softened, and coolin rapidly. If the aluminum sheetsare heate to a temperature substantially in excess of 450 C. the metalis rendered unfit for use in electrolytic cells. first efi'ect upon themetal as this temperature is exceeded is the formation of numer ousprotuberances looking like small blisters or warts upon the metaLsurfaceas though The Specification of Letters Patent. Patented July 23, 1918,

Application flied Kay 7, 1917. Serial No. 186,828.

" gas bubbles were forming in the metal.

Metal subjected to temperatures much higher than 450 C. is darkened incolor and has a peculiar glazed or varnished appearance, while theprotuberances are larger and more numerous. the new product resultingfrom the practice of my invention. In carrying out my invention, themetal 1s cast as usual into any shape suitable for subsequent mechanicaltreatment, such as iliawing, spinning, stamping, forging or the 1 (e.ment which is carried out as usual, the metal unavoidably becomeshardened. Thereupon the shaped or worked articles are placed in afurnace whichniay be heated and cooled quickly, for example, an electricmuffle furnace, and heated rapidly to a temperature in excess of 370 0.,and preferably below 450 C. The annealing treatment is continued forabout 2% to 3 hours. The aluminum articles then are removed from thefurnace and are quickly cooled, for example, if the articles are large,by blowing air upon them or by quenching them in water. Smaller articlesmay be sufiiciently rapidly cooled by merely exposing to the air.

'The formation of graphitic silicon appears to be dependent both on thetemperature and on the length of time the metal is subjected to thetemperature at .which graphitic silicon is formed. Apparently there is acritical temperature below 370 C. at which the transformation ofdissolved silicon into graphitic silicon takes place rapidly. Therefore,it is important in the cooling down of the softened metal, not to allowit to remain any, longer than necessary at the temperatures below 370where conversion takes place.

What I claim as new and desire to secure by Letters Patent of the Unitedbtates, is

1. The process of annealing aluminum rendered hard by mechanical workingand containing dissolved silicon which consists in subjecting the metaluntil softenedto a heat treatment above the temperature at whichdissolved silicon is converted into graphitic silicon, but below thetemperature at which the metal surface becomes uneven and darkened, andthen cooling the aluminum so rapidly to ordinary temperature that the.formation of appreciable graphitoidal silicon is avoided.

My invention also includes During the desired mechanical treat- 2. Theprocess of annealing aluminum containing dissolved silicon and which hasbeen rendered hard by mechanical Working, which consists in rapidlycarrying the metal 5 to a temperature within the range of about 370 C.and 450 C.,'maintaining a temperature Within said range until thealuminum is softened and then quickly chilling the metal, therebypreventing the formation of appreciable graphitoidal silicon at tempera-10 tures below said range.

In witness whereof, I have hereunto set my hand this third day of May1917. 5 LAURENCE H. WHITNEY.

